I propose to characterize biochemically a mouse Fc receptor and will use a previously isolated rat anti-mouse Fc receptor monoclonal antibody to prepare immunoadsorbent columns to purify the antigen recognized by this antibody. The Fab fragment of the monoclonal IgG blocks binding to mouse macrophages of mouse IgGl and IgG2b but not IgG2a immune aggregates and defines an antigen present only on Fc receptor bearing cells. This antigen is absent on Fc-receptor II negative variants of the J774 macrophage cell line. A set of monoclonal antibodies directed against the purified antigen will be prepared, which will be used in subsequent analysis of the different domains of the molecule and its orientation in the plasma membrane. The anti-Fc receptor monoclonal IgG and other monoclonal antibodies directed against macrophage plasma membrane proteins provide a tool of exquisite specificity for analysis of the interactions between different proteins and the dynamics of synthesis and turnover of these molecules. Our studies will focus on the mouse Fc receptor. By means of cleavable crosslinking reagents and immunonprecipitation techniques we hope to explore Fc receptor-protein interactions in resting cells and macrophages ingesting immune complexes. The theory that plasma membrane proteins are recycled after xendocytosis will be critically evaluated by immunological techniques are a recently developed method to label pinosome proteins. We propose further to isolate monoclonal antibodies directed against human Fc and C3b receptors, to analyze these poorly characterized but immunologically important receptors. Finally, anti-receptor antibodies may prove useful to analyze the role of the receptors in in vitro and in vivo systems, in addition to providing potentially valuable markers for diagnosis of lymphoproliferative diseases.